1. Field of Invention
Hydrogen atom donor compounds are useful as contrast enhancers when used in combination with (i) hindered phenol developers, and (ii) trityl hydrazide and/or formyl-phenyl hydrazine codevelopers, to produce ultra-high contrast black-and-white photothermographic and thermographic elements.
2. Background of the Art
Silver halide-containing, photothermographic imaging materials (i.e., heat-developable photographic elements) which are developed with heat, without liquid development have been known in the art for many years. These materials are also known as "dry silver" compositions or emulsions and generally comprise a support having coated thereon: (a) a photosensitive compound that generates silver atoms when irradiated; (b) a relatively non-photosensitive, reducible silver source; (c) a reducing agent (i.e., a developer) for silver ion, for example for the silver ion in the non-photosensitive, reducible silver source; and (d) a binder.
The photosensitive compound is generally photographic silver halide which must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver atoms (also known as silver specks, clusters, or nuclei) are generated by irradiation or light exposure of the photographic silver halide, those silver atoms are able to catalyze the reduction of the reducible silver source. It has long been understood that silver atoms (Ag.degree.) are a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions. The silver halide may be made "in situ," for example by adding a halogen-containing source to the reducible silver source to achieve partial metathesis (see, for example, U.S. Pat. No. 3,457,075); or by coprecipitation of silver halide and the reducible silver source material (see, for example, U.S. Pat. No. 3,839,049). The silver halide may also be made "ex situ" and added to the organic silver salt. The addition of silver halide grains to photothermographic materials is described in Research Disclosure, June 1978, Item No. 17029. It is also reported in the art that when silver halide is made ex situ, one has the possibility of controlling the composition and size of the grains much more precisely, so that one can impart more specific properties to the photothermographic element and can do so much more consistently than with the in situ technique.
The non-photosensitive, reducible silver source is a material that contains silver ions. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic materials, such as silver imidazolates, have been proposed. U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image is generally not visible by ordinary means. Thus, the photosensitive emulsion must be further developed to produce a visible image. This is accomplished by the reduction of silver ions which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms (i.e., the latent image). This produces a black-and-white image. In photographic elements, the silver halide is reduced to form the black-and-white image. In photothermographic elements, the light-insensitive silver source is reduced to form the visible black-and-white image while much of the silver halide remains as silver halide and is not reduced.
In photothermographic elements the reducing agent for the organic silver salt, often referred to as a "developer," may be any material, preferably any organic material, that can reduce silver ion to metallic silver. At elevated temperatures, in the presence of the latent image, the silver ion of the non-photosensitive reducible silver source (e.g., silver behenate) is reduced by the reducing agent for silver ion. This produces a negative black-and-white image of elemental silver.
While conventional photographic developers such as methyl gallate, hydroquinone, substituted-hydroquinones, catechol, pyrogallol, ascorbic acid, and ascorbic acid derivatives are useful, they tend to result in very reactive photothermographic formulations and cause fog during preparation and coating of photothermographic elements. As a result, hindered phenol reducing agents have traditionally been preferred.
As the visible image in black-and-white photothermographic and thermographic elements is usually produced entirely by elemental silver (Ag.degree.), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is often desirable to reduce the cost of raw materials used in the emulsion and/or to enhance performance. For example, toning agents may be incorporated to improve the color of the silver image of the photothermographic elements as described in U.S. Pat. Nos. 3,846,136; 3,994,732; and 4,021,249.
Another method of increasing the maximum image density in photothermographic and thermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming or dye-releasing materials in the emulsion. Upon imaging, the dye-forming or dye-releasing material is oxidized, and a dye and a reduced silver image are simultaneously formed in the exposed region. In this way, a dye-enhanced black-and-white silver image can be produced. Dye enhanced black-and-white silver image forming materials and processes are described in U.S. Pat. No. 5,185,231.
Thermographic imaging constructions (i.e., heat-developable materials) processed with heat, and without liquid development, are widely known in the imaging arts and rely on the use of heat to help produce an image. These elements generally comprise a support or substrate (such as paper, plastics, metals, glass, and the like) having coated thereon: (a) a thermally-sensitive, reducible silver source; (b) a reducing agent for the thermally-sensitive, reducible silver source (i.e., a developer); and (c) a binder.
In a typical thermographic construction, the image-forming layers are based on silver salts of long chain fatty acids. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. At elevated temperatures, silver behenate is reduced by a reducing agent for silver ion such as methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechol, pyrogallol, ascorbic acid, ascorbic acid derivatives, and the like, whereby an image of elemental silver is formed.
Some thermographic constructions are imaged by contacting them with the thermal head of a thermographic recording apparatus, such as a thermal printer, thermal facsimile, and the like. In such instances, an anti-stick layer is coated on top of the imaging layer to prevent sticking of the thermographic construction to the thermal head of the apparatus utilized. The resulting thermographic construction is then heated to an elevated temperature, typically in the range of about 60.degree.-225.degree. C., resulting in the formation of an image.
The imaging arts have long recognized that the fields of photothermography and thermography are clearly distinct from that of photography. Photothermographic and thermographic elements differ significantly from conventional silver halide photographic elements which require wet-processing.
In photothermographic imaging elements, a visible image is created by heat as a result of the reaction of a developer incorporated within the element. Heat is essential for development and temperatures of over 100.degree. C. are routinely required. In contrast, conventional wet-processed photographic imaging elements require processing in aqueous processing baths to provide a visible image (e.g., developing and fixing baths) and development is usually performed at a more moderate temperature (e.g., 30.degree.-50.degree. C.).
In photothermographic elements only a small amount of silver halide is used to capture light and a different source of silver (e.g., silver behenate) is used to generate the image. Thus, in photothermography the silver halide serves as a silver catalyst generator for the development of the non-photosensitive, reducible silver source. In contrast, conventional wet-processed black-and-white photographic elements use only one form of silver (e.g., silver halide) which, upon development, is itself converted to the silver image. Additionally, photothermographic elements require an amount of silver halide per unit area that is as little as one-hundredth of that used in conventional wet-processed silver halide.
Photothermographic systems employ a light-insensitive silver salt, such as silver behenate, which participates with the reducing agent in developing the latent image. In contrast, photographic systems do not employ a light-insensitive silver salt directly in the image-forming process. As a result, the image in photothermographic elements is produced primarily by reduction of the light-insensitive silver source (silver behenate) while the image in photographic black-and-white elements is produced primarily by the reduction of silver halide.
In photothermographic elements, all of the "chemistry" of the system is incorporated within the element itself. For example, photothermographic elements incorporate a developer (i.e., a reducing agent for the non-photosensitive reducible source of silver) within the element while conventional photographic elements do not. The incorporation of the developer into photothermographic elements can lead to increased formation of "fog" upon coating of photothermographic emulsions as compared to photographic emulsions. Even in so-called instant photography, developer chemistry is physically separated from the silver halide until development is desired. Much effort has gone into the preparation and manufacture of photothermographic elements to minimize formation of fog upon coating, storage, and post-processing aging.
Similarly, in photothermographic elements, the unexposed silver halide inherently remains after development and the element must be stabilized against further exposure and spontaneous development. In contrast, the silver halide is removed from photographic elements after development to prevent further imaging (i.e., the fixing step).
In photothermographic elements the binder is capable of wide variation and a number of binders are useful in preparing these elements. In contrast, photographic elements are limited almost exclusively to hydrophilic colloidal binders such as gelatin.
Because photothermographic elements require thermal processing, they pose different considerations and present distinctly different problems in manufacture and use. In addition, the effects of additives (e.g., stabilizers, antifoggants, speed enhancers, sensitizers, supersensitizers, etc.) which are intended to have a direct effect upon the imaging process can vary depending upon whether they have been incorporated in a photothermographic element or incorporated in a photographic element.
Distinctions between photothermographic and photographic elements are described in Imaging Processes and Materials (Neblette's Eighth Edition); J. Sturge et al. Ed; Van Nostrand Reinhold: New York, 1989; Chapter 9 and in Unconventional Imaging Processes; E. Brinckman et al, Ed; The Focal Press: London and New York: 1978; pp. 74-75.
Hydrazides have been used in conventional wet processed black-and-white and color photographic systems. They have found use as nucleating agents, infectious developers, contrast and speed improving agents, and color developing agents.
Hydrazides have been studied as infectious developers for use in photographic graphic arts films. See U.S. Pat. Nos. 4,798,790 and 4,925,832 and Kitchin, J. P. et al. J. Photogr. Sci. 1987, 35, 162-164 and Kitchin, J. P. et al. J. Imag. Technol. 1989, 15(6), 282-284.
The use of hydrazides with hydroquinone-type (e.g., polyhydroxy benzene) developers is known to produce photographic elements with very high contrast in wet silver halide photographic materials. These materials have been found to be very advantageous for graphic arts laser imaging applications.
U.S. Pat. Nos. 4,891,295; 4,902,599; 4,952,474; 4,970,132; 5,037,719; 5,070,132; 5,196,288; and 5,246,811 all refer to the use of formyl-phenyl hydrazines and/or their derivatives in combination with traditional wet silver halide-type developers; silver halide; a polymerizable compound, and a dye or color-forming compound (sometimes contained in microcapsules). Upon imaging and heat development of the light-sensitive material, the hydrazide initiates polymerization within the area where the latent image has been formed. Pressure is then applied to transfer the dye image from the unpolymerized capsules to a receiving sheet. Silver soaps are optionally included and incorporation of the polymerization and dye-forming materials into microcapsules is preferred. In these systems, the hydrazide is an initiator for the polymerization while the developer develops the silver halide.
U.S. Pat. No. 4,735,884 uses trityl hydrazides in a gelatin-based construction as a reducing agent for silver halide as well as an initiator for the polymerization of acrylates within the area where the latent image has been formed. The hydrazides are encapsulated with other traditional silver halide developers, the photosensitive silver halide and leuco dyes or pre-formed dyes. After exposure, heat is applied to accelerate the crosslinking reactions, pressure is applied to break the non-exposed capsules, and the positive color image is transferred to a receiving layer.
U.S. Pat. No. 4,772,531 combines a sulfonamidophenol developing agent moiety and a trityl hydrazide group in one molecule to minimize the need for an additional developer within the polymerization system.
U.S. Pat. No. 3,667,958 uses a combination of a polyhydroxybenzene reducing agent (i.e., a hydroquinone-type developer) with a hydroxylamine, reductone and/or a hydrazine reducing agent in photothermographic constructions to produce black-and-white images. A preferred hydrazine is 1-formyl-2-phenyl hydrazine.
U.S. patent application Ser. No. 08/369,730 (filed Jan. 6, 1995) discloses hydrazide compounds useful as co-developers for black-and-white photothermographic elements. These elements contain (i) a hindered phenol developer, and (ii) a trityl hydrazide or a formyl phenylhydrazine co-developer, and provide elements having high Dmax (&gt;5.00), fast photospeeds, and high contrast (&gt;20.0). It would be advantageous to improve the reactivity of these compounds by the addition of a contrast-promoting agent. This would allow the reduction in the amount of silver as well as the amount of hydrazine or hydrazide co-developer needed to achieve high contrast, lower costs, and improve the reactivity of less effective compounds. It would be especially desirable to be able to achieve in dry photothermographic or thermographic elements the high contrast that is currently available in wet silver halide materials. New developing agent systems for use photothermographic and thermographic elements are therefore desired.